Straight answers about tank life, BTUs, and knowing how much gas you have left.
It comes down to two numbers: how much propane is in the tank and your grill's total BTU output. A full 20 lb tank holds about 432,000 BTU. Divide that by your grill's BTU/hr — 30,000 gives about 14 hours, 40,000 about 10–11 hours, 60,000 about 7 hours at full blast. The calculator does this for any tank and grill.
About 21,594 BTU per pound. That is roughly 91,500 BTU per US gallon, and a gallon of propane weighs about 4.24 pounds. So a full 20 lb tank carries around 432,000 BTU of energy.
Weigh it. Put the tank on a bathroom scale and subtract the tare weight (TW) stamped on the collar — usually about 17.2 lb on a 20 lb tank. The difference is pounds of propane left. The warm-water trick shows the fuel level (pour warm water down the side and feel for the cool line) but not the exact pounds.
Add up every burner's rating. Two-burner grills are often 26,000–30,000 BTU/hr, three-burner grills 36,000–45,000, and larger four to six burner grills 50,000–75,000. Side and sear burners count too when lit. Check the plate under the lid or your manual.
Yes. In cold temperatures tank pressure drops, so the grill may not hit its full rated output, and you tend to run burners harder to hold heat. The energy in the propane is unchanged, but real cook time can be shorter on a freezing day.
Refilling is usually cheaper per pound. Exchange tanks are often filled to about 15 lb rather than a full 20 lb, so you pay more for less gas — but exchange is fast and everywhere. Refill stations fill to the tank's real capacity, giving you more grilling time per dollar.
Plug in your tank size, fuel level, and grill BTUs to see your exact hours of grilling left.